1. Field of the invention
The present invention relates to an electrostatic ink jet recording apparatus and, in particular, to such an electrostatic ink jet recording apparatus, in which printing density can be easily controlled and appropriate printing density can be easily obtained.
2. Description of the Prior Art
Ink jet technology is widely used as low-cost, high-quality and high-speed printing technology in information recording apparatuses for office automation and individuals such as copying machines, facsimile machines, printers, word processors and the like.
With respect to principles of ink jet recording, energy generation means for flying a coloring material includes means making use of a heating resistor such as an electrothermal conversion device, means making use of a piezoelectric element such as an electromechanical transducer, and electrostatic means making use of electric energy as it is.
Among them, an electrostatic ink jet recording apparatus has a recording head, which can be produced easier than other systems and can perform gradation recording by controlling an electric signal applied to a recording electrode. In addition, since a current consumed at the time of recording is remarkably small, it can be said that the electrostatic ink jet recording technology is one that is useful as energy-saving equipment also in future. Furthermore, this technology uses an oil based pigment dispersed ink to enable performing printing, which is excellent in water resisting property, and in particular, is highly useful for office automation.
Here, a principle aspect of the electrostatic ink jet will be briefly described. As proposed in Japanese Patent Unexamined Publication No. 56-4467, when a recording voltage of several kilovolts is applied between a recording electrode filled with ink and a counter electrode holding a recording medium, and then exceeds a certain threshold value, an electrostatic force acting on the ink overcomes the surface tension of the ink, so that ink droplets are made to eject and fly from the recording electrode to the counter electrode.
Structure of a head in accordance with this principle is disclosed in, for example, U.S. Pat. No. 4,271,416 and Japanese Patent Unexamined Publication No. 56-4467. With structures in these examples, as shown in FIG. 11, any nozzle hole is not necessarily required for each recording electrode, but an ink ejection portion may be of so-called slit type to be common to a plurality of recording electrodes. This type of structure has a feature in dispensing with a nozzle or nozzles to thereby reduce clogging due to drying of ink. Therefore, with this arrangement, several hundred of recording electrodes are provided to be greatly effective not only for a so-called serial type head, which scans in a widthwise direction of a recording medium, but also for a so-called full-line type head having a recording electrode extending over the width of the recording medium.
In this type of electrostatic ink jet, in order to actuate respective recording electrodes independently, it is necessary to prevent leakage of electric charges, which each recording electrode may make a closed loop with an adjacent electrode to cause. Therefore, an oil based solvent having high electrical resistance is usually used as a solvent for ink. An example shown in Japanese Patent Unexamined Publication No. 58-215353 uses an oil based ink having specific resistance of approximately 108 xcexa9xc2x7cm, surface tension of 18 dyne/cm, viscosity of 2-30 cP, and specific gravity of 1.0 g/cm3. However, the oil based solvent is lower in surface tension than a water solvent (having surface tension of about 70-80 dyne/cm) is. Therefore, the oil based solvent is problematic in that, when printing is performed, both particles of a coloring material and the solvent rapidly permeate into fibers of recording paper to cause reduction of printing density, bleeding and offset. This is because the particles of coloring material and an oil carrier liquid that is a solvent for the particles are made to fly at the same time.
In contrast, as disclosed in Japanese Patent Unexamined Publications Nos. 8-295023 and 9-193389, it has been attempted to solve this problem by applying the electrostatic force only to the particles of coloring material from the ink. Although the ejecting principle is not necessarily elucidated in these examples, the above-described problem is solved because an electrostatic force is made to act on only the particles of coloring material to fly only the particles of coloring material, so that the carrier fluid is not included in ejected ink. According to the contents of Japanese Patent Unexamined Publication No. 9-193389, the use of an ink having the specific resistance of 1010 xcexa9xc2x7cm or higher makes printing density very high to provide a particularly desirable effect in sharpness of contour.
However, since no or exceedingly little spreading into a recording medium is caused when only the particles of coloring material are ejected, the optical density of printed dot becomes too high, so that an optical reflectance of a printed image will decrease remarkably. As a result, brightness of a color image reduces whereby a defect is newly found that a color reproduction range becomes narrow to provide a generally dark image quality.
Furthermore, in the prior technology, the principle of ejecting only the particles of coloring material from the ink causes high density of ink at a tip of the recording electrode to increase the viscosity of the ink, thus presenting a problem that ejecting speed decreases and ejecting fails due to drying of the ink. In addition, ejecting of only the particles of coloring material makes the coloring material attaching to the recording medium nearly solid. Therefore, fixing of such coloring material necessitates a fixing process by means of temperature or pressure, which is problematically costly.
It is an object of the present invention to obtain an appropriate printing density of coloring material on a recording medium and to solve problems of the prior art in an electrostatic ink jet such as excessive reduction of printing density, bleeding, offset and the like as well as problems, caused by excessively increased density, such as reduction in color reproduction range and in recording speed, and cost increase caused by high-temperature fixing process.
To solve the above-described problems, the inventors of the invention have earnestly studied geometric dimensions of an ink meniscus in order to obtain an electrostatic ink jet head, which provides an optimum printing density and reproduces optimum colors, and is excellent in response and fixing properties. As a result, it has been found that, in the case where an angle of an ink meniscus relative to an electrode at a tip of a recording electrode is 0-65xc2x0 or a distance of the ink meniscus from the tip of the electrode is 0-1500 xcexcm, any value of 0.8 to 2.0 can be obtained for optical density to enable controlling color, response and fixing properties.